The present invention relates to an anodized memory disk substrate and method of manufacturing the same. In particular, the invention provides the anodized memory disk substrate, wherein the rate of growth of anodic film is fast at a relatively low voltage, the hardness of film is high, and the head crush resistance and further the heat resistance are excellent.
The memory disks made from aluminum or aluminum alloy are used conventionally and so-called coated type disks, are manufactured by coating with magnetic film. Recently, with the use of high densification of the recording, processes of the plating-plating type, plating-sputtering type, anodizing-sputtering type, etc. have attracted attention. In these cases, the plating layer or the anodic film compensates for the surface hardness of the aluminum or aluminum alloy.
And, for such anodic film, three severe requests must be met:
(a) No occurrence of cracks by heating, that is, high heat resistance. PA1 (b) High hardness. PA1 (c) No defects such as pits etc. on the surface.
The heat resistance aforementioned is necessitated at the time of the formation of thin-film magnetic recording medium, and this is because of being heated at 350.degree. C. in order to convert .alpha.-Fe.sub.2 O.sub.3 to .gamma.-Fe.sub.2 O.sub.3 via Fe.sub.3 O.sub.4.
The hardness is a principal purpose of the film. If the surface hardness is low, the substrate is dented due to the collision of magnetic head, that is, the head crush resulting in the damage of magnetic recording medium.
Moreover, if there exists defects such as surface pits etc., recording errors are generated unpreferably.
Conventional anodic film prepared using sulfuric acid is most suitable since the magnetic stability is excellent from the point of not carrying the magnetism and the corrosion resistance is also excellent as well as the hardness and the finish obtained by polishing is also good. But, it has a significant shortcoming that cracks occur by heating and, if cracks occur, innumerable recording errors are generated. For this reason, treatment methods, by which the cracks do not occur, have been investigated hitherto and the anodizing treatment in chromic acid solution was proposed (refer to, for example, Japanese Unexamined Patent Publication No. Sho 59-171023 and No. Sho 59-180832).
However, in preparing anodic film by means of chromic acid, the hardness is still insufficient and there is a difficulty in the treatment property thereof. Namely, there is a shortcoming in that the rate of growth of film is slow and, if attempting to raise the current density, high temperature and high voltage are needed and the hardness of film becomes insufficient.
As a result of intensive studies, an anodized memory disk substrate and method of manufacturing the same have now been developed by the present invention, wherein conventional shortcomings as described above are dissolved, the rate of growth of film is fast at a relatively low voltage, the anodic film has high hardness and is excellent also in the heat resistance, and further the head crush resistance is excellent from the point of hardness.
The anodizing treatment by means of only chromic acid solution is carried out most frequently hitherto as described above and is utilized for airplane members. In this case, the thickness of film is generally not more than several .mu.m and is extremely thin. In order to make this film thicker, it is necessary to increase the current density and prolong the electrolytic time. Since the chromic acid solution aforementioned shows low electroconductivity, the solution temperature should be raised or the electrolysis should be conducted at high voltage to raise the current density. As a result of diligent studies, the inventors have found that, through treatment with a chromic acid-sulfuric acid mixture, by adding sulfuric acid to chromic acid solution, a film being thick and excellent in heat resistance and hardness is formed at relatively low temperature and low voltage in a short time. In consequence of further studies, the invention has been completed.